Generally, single crystal ingots have been grown by the Czochralski method (CZ method). In the CZ method, first, a mass of polycrystalline silicon filled in a crucible made of quartz is melted by heating using a heater to obtain a melt of silicon. Then, a lower end portion of a seed crystal attached to a seed holder is dipped into the silicon melt, and the seed holder and crucible are rotated while the seed holder is gradually pulled upward for causing a single crystal silicon ingot to grow at the lower end of the seed crystal.
Meanwhile, in the seed crystal, dislocation is generated by a thermal shock when the seed crystal is dipped into the melt. For eliminating this dislocation, a Dash necking process has been adopted. In the Dash necking process, the seed crystal after dipping is narrowed to be small in diameter to continuously form a neck portion on the order of a diameter of 3 mm from the seed crystal so as to release the dislocation to the front surface of the neck portion. After thus making the ingot free from dislocation, the pulling speed of the seed crystal is appropriately reduced, and a shoulder portion and straight trunk portion of the ingot are pulled up in sequence.
Recently, single crystal silicon ingots have been produced with larger diameters and longer lengths in order to improve the productivity and yield of devices. As a result, the ingots have been increased in weight, and there has been a possibility that the neck portion may rupture in the case of pulling up a single crystal ingot produced only with the conventional Dash necking process, which prohibits safe ingot growth.
Therefore, as a conventional art for solving this problem, for example, Patent Literature 1 is known. Patent Literature 1 is for forming, directly under the above-mentioned neck portion, an engaging portion in which an increased diameter portion and a reduced diameter portion (a constricted portion) continue, and by mechanically gripping the engaging portion by gripping members of a holding mechanism, for shifting the load of an ingot from the neck portion to the gripping mechanism to supplement insufficiency in strength of the neck portion.
Moreover, when growing a single crystal ingot, due to fluctuation in melt temperature and variation in accuracy of diameter control, it is impossible to make the shape of the outer peripheral surface of an ingot completely match a target shape. Therefore, on the outer peripheral surface of a formed engaging portion, annular unevenesses that are different in shape exist. Accordingly, contact between the gripping members and engaging portion results in point contact or line contact with a short contact width, and as a result of concentration of stress per unit area with respect to the ingot, rupture and dislocation of the ingot at a gripped location of the engaging portion have been generated. In the case of complete rupture of an ingot during single crystal ingot growth, this may have lead to a major accident of an ingot fall.
In order to solve this problem, in Patent Literature 1, there are provided at a contact part of the gripping members with the engaging portion attachment members made of a heat resistant soft material that are plastically deformed when receiving an ingot load. As a result of being plastically deformed, the attachment members are closely fitted to uneven parts of the ingot. Accordingly, the contact area between the gripping members and engaging portion is increased, and stress concentration per unit area with respect to the ingot is relieved.